1. Field of the Invention
This invention relates to circuit breakers and, more particularly, it pertains to circuit breakers having a remotely controlled electromagnetic solenoid and functions both as a current limiting circuit breaker and contactor with a single set of contacts that is operated manually, by a bimetal, by a short circuit trip coil, or by a solenoid-bistable device.
2. Description of the Prior Art
In recent years, electrical distribution systems have increased in size and capacity to meet expanding demands of electrical service. Utilities have adopted lower impedance transformers to reduce system power losses, regulation problems, and cost. But the short circuit fault currents available to plaque distribution systems continue to increase, reaching as high as 200,000 A.
To prevent these high available fault currents from damaging electrical distribution systems, protective devices limiting the perspective let-through currents are required. Fuses and, more recently, current limiting circuit breakers, have been used successfully to limit these fault currents. They can reduce, to tolerable levels, both the peak fault currents (I.sub.p) and thermal energy (I.sup.2 t) that reach downstream equipment. Mechanical and magnetic forces that can destroy equipment are proportional to the square of the peak currents (I.sub.p).sup.2, and thermal damage is proportional to the energy let-through (I.sup.2 t).
Large short circuit currents result from the use of low impedance transformers and interconnected networks in modern low voltage AC power distribution systems. Fault currents in excess of 100 KA are common. Traditionally, high fault current prediction has been provided in current limiting fuses in conjunction with circuit breakers. However, a new generation of high speed electromagnetically driven, single, and multiple break current limiting devices have been developed. These devices not only perform the function of a circuit breaker and current limiting fuse, but are also resettable and reusable. These devices can also be effectively applied to motor control as well as power distribution systems.
Associated with the forgoing is a growing need for electronic means for communication and control in electrical distribution systems. For that purpose, circuit breakers operated by remotely controlled electromagnet means, such as by a solenoid, have been employed. One disadvantage of some types of these circuit breakers has been a requirement of continued power to keep the contacts closed. Here the tripping time could be delayed because of the time required to collapse the flux in the solenoid and open the contacts.
Another disadvantage of some prior circuit breakers has involved the safety of personnel. Some prior circuit breakers could be actuated by remote control to an "on" or closed circuit condition, even through the breaker had been previously tripped to an open circuit by a person on-site for some purpose such as maintenance.